Author:

Chris Crabtree(U.S. Naval Research Laboratory)

Wave particle interactions between electrons and whistler waves are a
dominant mechanism for controlling the dynamics of energetic electrons in
the radiation belts. They are responsible for loss, via pitch-angle
scattering of electrons into the loss cone, and energization to millions of
electron volts. It has previously been theorized that large amplitude waves
on the whistler branch may scatter their wave-vector nonlinearly via
nonlinear Landau damping leading to important consequences for the global
distribution of whistler wave energy density and hence the energetic
electrons. It can dramatically reduce the lifetime of energetic electrons in
the radiation belts by increasing the pitch angle scattering rate. The
fundamental building block of this theory has now been confirmed through
laboratory experiments. Here we report on in situ observations of wave
electro-magnetic fields from the EMFISIS instrument on board NASA's Van
Allen Probes that show the signatures of nonlinear scattering of whistler
waves in the inner radiation belts. In the outer radiation belts, whistler
mode chorus is believed to be responsible for the energization of electrons
from 10s of Kev to MeV energies. Chorus is characterized by bursty large
amplitude whistler mode waves with frequencies that change as a function of
time on timescales corresponding to their growth. Theories explaining the
chirping have been developed for decades based on electron trapping dynamics
in a coherent wave. New high time resolution wave data from the Van Allen
probes and advanced spectral techniques are revealing that the wave dynamics
is highly structured, with sub-elements consisting of multiple chirping
waves with discrete frequency hops between sub-elements. Laboratory
experiments with energetic electron beams are currently reproducing the
complex frequency vs time dynamics of whistler waves and in addition
revealing signatures of wave-wave and beat-wave nonlinear wave-particle
interactions. These new data suggest that these weak turbulence processes
may be playing a role in saturating the nonlinear instability.

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2016.DPP.TI3.5